In seismic exploration, proposed prescriptions for shooting (or vibrating) and collection operations must be followed--precisely--in the field. Such is particularly important in field techniques involving so-called Common Depth Point Recording (or CDPR) operations in which changing sets of sensors were used in association with successive shots to provide multiple stacked recordings. (In all of the following teachings, the words "shots" and "shooting" will be used for the part of the operation in which the sound waves are generated and sent down into the sub-surface. It will be appreciated, however, by those skilled in the art, that the same teachings apply where the sound waves are generated by large vibrators at the surface rather than by explosive shots.)
In the CDPR process, sensors and energy sources are positioned at a first series of spatially (geometrically) related locations, to produce a first record. Subsequent records are then made with the sensors and energy source occupying new locations. However, the sensors and energy source normally maintain a similar spatial relationship to each other.
Advancement of the source sensor locations (in CDPR operations) employ a technique commonly known as "rollalong". In the rollalong technique the sensor array is not physically lifted up and relocated for every advancement in position. Instead, a larger array of sensors than is needed at any one time is laid down along a line of survey, and subsets of that larger array (called the "active" array) are selected one at a time, for seismic collection purposes, progressing along the larger array. Relative advancement of the subsets is commonly done in a very rapid manner using a switching device called a rollalong switch (such as shown, e.g., in my U.S. Pat. No. 3,618,000, Nov. 2, 1971 for "Rollalong Switch",), in which a large number of sensors can be controllably provided at any programmed interval along the recording line. Through the use of multiple pair cables extending along the line, these sensors are connected into the input receptables of the rollalong switch. The design of the switch permits the "active" array of sensors to be interconnected to the input of the geophysical seismic recorder and keeps track of the position of one end of the "active" array relative to the number of recording channels available (say any position within 1-in-60 channels). Not only can the switch select any contiguous group of sensors, from the total number positioned along the line, but it can also add gaps in the active spread using one or more "inactive" groups as the gapping members. This technique is usually used when a large energy source is positioned at the center of the active array, because the horizontal energy received at the near sensors would be so strong that it would mask out the weaker reflected energy that is of primary interest. (A "gapped" array for a 24-sensor record, for example, consists of sensors 1 through 12 and 15 through 26 with sensors 13 and 14 left disconnected by the rollalong switch).
Computer processing of the CDPR field data (now commonly done by large centralized computer facilities) requires not only the actual time versus amplitude seismic reflection data (recorded on field tapes), but also requires "housekeeping" data describing associated source and sensor geometry, as the former was collected. These latter data consist of, inter alia, positional locations for every sensor in each "active" array during the collection sequence, the location of the energy source, and the location and size of the gap (if any). To provide the above, the usual field procedure is to determine the ground locations by survey prior to the recording operations. The location and direction of the line is referenced to know geographic locations or geodetic survey points. The location of each sensor (or sensor group) and energy source is surveyed in and marked with a survey stake having an identification number representing a ground location. These locations are written down in the survey log for that particular seismic line. The surveyor's log thus contains part of the geometrical data that must be added to the seismic data after the latter has been recorded and is ready for processing.
Another requirement of the geometrical data is developed during the recording process. It relates to data entered into the "observer's" log. (The operator of the seismic recording system is commonly called the "observer"). The observer's log contains, for each record sequence, the spatial extent of the active sensors usually identified by ground locations of the sensors at each end of the active array. In the event that the active array contains a gap, the location of the gap will be specified relative to adjacent active sensor locations. The observer's log also contains the location of the energy source for each record. In some cases, when a pre-surveyed line is recorded, the energy source cannot be located at the location designated for it during the survey. In these situations, the shot location from the observer's log must be used in processing instead of the original survey data. The observer's log also contains information that infers or describes spatial irregularities in the active array imposed by field conditions when the line is recorded.
As previously indicated, the rollalong switch tracks the position of active array (for identifying the location of the "active" sensor array including gap). While some rollalong switch units provide for transferring such array information directly to the field recorder (recognizable as header data on the digital seismic tape) these data are not in terms of true ground location but in an arbitrary numbering sequence relative to a particular recording vehicle location. The true ground location of the recording vehicle must therefore be entered into the observer's log in order to convert rollalong switch positions to true ground locations.
The foregoing description of geophysical seismic data recording operations indicates conclusively, that collection of seismic reflection data must be supported by accurate and sufficient correlative data so as to accurately define spatial source and sensor geometry relative to a permanent geographical location. It also indicates that separate types of cross-checking materials, for documentation, are needed as the data is collected including the steps of generating, formatting and displaying spread and sound geometries for both present and next-in-time shooting and recording sequences.